Today’s EagleFiler 1.8.14 update announcement mentions this improvement:
“The Apple Mail importer is better able to reassemble messages in cases where Mail has saved duplicate copies of an attachment
file in the same folder.”
Does this work retroactively on messages already imported?
Otherwise, how did the problem it solves manifest itself?
No. It seems to happen when a message has an attachment with non-ASCII characters in its name. Mail saves duplicate copies of it to disk, which confused previous versions of EagleFiler because it was only expecting to find one file for each attachment.